Manufacture of flat glass on improved molten metal bath



Sept. 19, 1967 c. JEWELL 3,342,574

R. MANUFACTURE OF FLAT GLASS 0N IMPROVED MOLTEN METAL BATH Filed May 13, 1964 .corporating a noble metal in a United States Patent This invention relates to the manufacture of flat glass in ribbon form.

In the specification of U.S.A. Patent No. 2,911,759 there is disclosed the employment of molten metal as means for supporting rolled glass on its way to a lehr, and there is also disclosed the employment of a bath of molten metal as a means of effecting superficial melting of the ribbon surface and for cooling the surface, whereby a fire polish is imparted to the ribbon.

In U.S.A. Patent No. 3,083,551 glass is delivered onto a bath of molten metal at a controlled rate and there is established on the bath a layer of molten glass from which is developed fiat glass in ribbon form having a fire polish.

In both specifications a bath of molten metal is described having the following characteristics:

C. approximately. C. (except where the bath The molten bath is constituted in its preferred form, of tin alone or of an alloy of tin and another metal in which alloy tin predominates, and in order to prevent contaminants being present at the interface between the glass and the bath a plenum of protective atmosphere is maintained in the headspace over the bath to exclude ambient atmosphere from the headspace.

In accordance with the present invention flat glass in ribbon form is advanced over a molten alloy, and from this aspect the present invention comprises advancing the glass over a molten alloy of tin and a noble metal in the manufacture of flat glass in ribbon form which noble metal forms at the operating temperature an oxide which is much less thermodynamically stable than an oxide of tin.

By the expression noble metal is meant one of the metals silver, gold, platinum, palladium or rhodium, or an alloy of two or more of the said noble metals, e.g. a silver/gold alloy.

The present invention also comprises advancing the glass in ribbon form over a molten alloy of tin and a noble metal, which alloy includes a proportion of a noble metal, which alloy includes a proportion of a noble metal such that the alloy has a melting point below 700 C and from this aspect the present invention comprises 'a inbath of molten tin in a proportion to form a tin alloy having a melting point below 700 C., which noble metal forms at the operating temperature an oxide which is less thermodynamically stable than an oxide of tin.

Preferably in accordance with the present invention silver is incorporated in the molten alloy, the presence of which silver modifies the entry of contaminants into the glass at the interface between the glass and the molten metal, thereby minimising any possibility of harmful contaminants entering the glass; while the tin component ensures the desired low melting temperature for the bath as well as the other desirable characteristics for the bath hereinbefore defined.

The presence of only 1% of silver in the molten alloy of tin and silver over which the fiat glass in ribbon form glides, has been found to eifect a useful decrease in the entry of harmful contaminants into the glass.

Further, experiments have disclosed that if the flat glass is in contact with a molten alloy of tin and 10% of silver, the ultimate fiat glass produced can subsequently be reheated and then chilled in a process of toughening glass, and the resultant product is uniformly transparent.

Also in accordance with the invention the molten alloy can be suitably constituted by using 30% of silver.

However in accordance with the invention the molten alloy may contain silver and tin in equal proportions as even in that case there is sufficient tin to achieve a sufficiently low melting temperature (in this case the melting point of the bath is about 500 C.) for the production of flat glass in continuous ribbon form and the reduction of the tin content to 50% materially assists in minimising the possibility of harmful contaminants entering the glass.

In order that the invention may be more clearly understood the embodiment thereof in a method of operating according to U.S.A. Patent No. 3,083,551 will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which:

FIGURE 1 is a sectional elevation of rolling means for converting molten glass into a ribbon of glass for delivery on to a bath of molten alloy contained in a tank structure, and

FIGURE 2 is a sectional elevation showing a modified method of delivering glass to the bath of molten lalloy.

In the drawings like references designate the same or similar parts.

In the construction shown in FIGURE 1 there is illustrated a forehearth of a continuous glass melting furnace, indicated at 1, a regulating tweel at 2, and a spout at 3. The spout comprises a lip 4 and side jambs 5, of which only one is illustrated. The side -jam bs 5 and lip 4 together form a spout of generally rectangular cross-section. A cover may be secured over the spout.

Operatively associated with the spout are a pair of water cooled casting rolls indicated at 6 and 7. These rolls are mounted on frames 8 in the usual manner and are driven by power means, not shown. A gate 9 is ad justably suspended in known manner, by means not shown, in a vertical plane in contig-uity with the upper casting roll 6. The gate 9 shields the top roll 6 from heat radiated by molten glass 10 flowing from the forehearth over the lip 4 to the pass between the rolls 6 and 7.

The upper casting roll 6 is disposed in advance of the lower casting roll 7 so that the molten glass flows from the lip 4 on to an upper part of the roll 7 presenting to the glass a downwardly and forwardly directed arcuate casting bed moving in the same sense as the direction of flow from the spout. Thus the molten glass on leaving the spout and arriving on this casting bed is constrained to flow forwardly, thereby preventing backward flow of the molten glass leaving the spout 4.

The conventional ribbon forming means, just described, is disposed over the floor 11 of a tank structure also including integral side walls :12 and integral end walls 13 and 14 respectively situated at the inlet and outlet ends of the tank structure.

The tank structure contains a bath 15 of a molten alloy of tin and a noble metal, preferably silver, for example a bath of molten tin containing 10% of silver. The bath has all the characteristics hereinbefore defined of the bath described in U.S.A. Patents Nos. 2,911,759 and 3,083,551. The surface level of the bath of molten alloy is indicated at 16.

The tank structure supports a roof structure including a roof '17 and a vertically adjustable end wall at each end of the roof. The adjustable end wall near the casting rolls is indicated at 18, and the adjustable end wall at the outlet end of the tank structure is indicated at 19. The roof structure provides a tunnel over the "bath 15 and defines a headspace 20 thereover.

The adjustable end wall 18 of the roof structure forms part of an enclosure for the casting rolls 6 and 7. This enclosure has a ceiling 21 and side walls 22, which side walls 'are carried by the side walls 12 of the tank structure. The ceiling 21 extends between the regulating gate 9 and the end wall '18.

1A protecting atmosphere, such, for example, as disclosed in said Patents 2,911,758 and 3,083,551, is maintained at a plenum in the headspace 20 over the bath 15, the protecting atmosphere being supplied through a series of ducts 23 connected by branches 24 to headers 25. Further, the passage of protecting atmosphere under the end wall 18 into the enclosure housing the rolls '6 and '7 ensures that the rolls operate in that atmosphere, thereby protecting the rolls from corrosion on their surfaces. The existence of a plenum in the headspace 20 and the enclosure for the casting rolls 6 and 7 prevents the ingress of ambient atmosphere.

The temperature of the molten bath '15 is regulated from the inlet end to the discharge end by providing thermal regulators 26 immersed in the bath. Radiant heaters 27 are provided in the headspace 20 to assist the temperature regulation. The bath may be maintained at such a temperature, e.g. 650 C., that it is not hot enough to melt the surfaces of the rolled ribbon of glass 28 as the ribbon glides along the bath; or the temperature of the bath can be regulated so that the ribbon of glass 28 is superficially melted as it glides along the bath and is then cooled before it is discharged from the bath to a state in which it can be taken off the bath unharmed by mechanical means, the ultimate ribbon of glass having a lustre of fire finish quality.

Outside the discharge end of the tank there are mounted conveying rollers 29 which are disposed somewhat above the top of the end wall 14 of the tank structure. The ribbon of g ass 28 is taken-up off the bath by the rollers 29 and conveyed to a conventional tunnel lehr, not shown.

Impurities, for example oxygen and/or sulphur may be present in the protective atmosphere over the bath of molten alloy or may migrate into the bath from the glass delivered to the bath. This may result in the presence of contaminants in the bath. The presence of a noble metal in the bath, according to the present invention, for example silver which may be present in a proportion which is as low as 1%, modifies the entry of contaminants into the glass as the ribbon glides over the molten alloy bath 15, thus minimising the entry of harmful contaminants into the glass.

Protection for the bath has been found to be particularly desirable in the region where the ribbon of glass 28 is taken-up off the bath surface by the conveying rollers 29. The use, according to the present invention, of a bath of a molten alloy of tin and a noble metal for conveying the cooled ribbon 28, minimises the entry of harmful contaminants into the glass just before the ribbon of glass leaves the bath of molten alloy.

A modified process of producing flat glass is shown in FIGURE 2. Glass in molten form is supplied to the bath from the spout 4, of the forehearth 1, and the gate 9 affords a constant regulation to control the flow of molten glass 60 from the spout lip 4; thus the flow is control-led by the regulating tweel 2 and the gate 9. The spout lip 4 is vertically spaced from the surface of the 4- bath so that the molten glass has a free fall of a few inches to the bath, which distance is such as to ensure a heel 31 being formed behind the glass flowing to the bath, which heel extends rearwardly under the spout lip 4 to the end wall 13.

The temperature at the inlet end of the bath is maintained at about 1000 C. or slightly higher over a sufficient length of the 'bath to ensure that a buoy-ant layer of molten glass 32 is established on the bath from which layer is developed a buoyant body of molten glass 33 by permitting the buoyant layer to how laterally unhindered to the limit of its free flow, the width of the tank structure being somewhat greater than the width of the buoyant body 3 3 of molten glass at the surface level 16 of the molten alloy bath.

The presence of the noble metal in the bath 15 ensures that the entry of contaminants into the glass is minimised as the buoyant body of molten glass 33 is developed and is advanced in ribbon form along the bath, which ribbon is cooled on the bath until it is sufficiently stiffened to be taken off the 'bath unharmed by the rollers 29, the outlet end of the tank structure being identical to the outlet end of the tank structure illustrated in FIGURE 1.

Experiments have shown that when carrying out the process of producing flat glass in accordance with U.S.A. Patent No. 3,083,551, but using a bath of a molten alloy of tin and a noble metal constituted as described above, the thickness of the glass formed on the molten alloy bath, which bath is kept free from contaminants, is sub stantia-lly the same as that produced on a bath of molten tin.

I claim:

1. In the manufacture of fiat glass in ribbon form, advancing the glass over a molten alloy of tin and a noble metal selected from the group consisting of silver, gold, platinum, palladium and rhodium, which noble metal is present in amounts from about 50% to about 1% of the bath and in a proportion to forma tin alloy having a melting point below 700 C., and forms at the operating temperature an oxide which is much less thermodynamically stable than an oxide of tin and maintaining a plenum of protective atmosphere over the molten alloy.

2. In the manufacture of flat glass in ribbon form, advancing the glass over a molten alloy of tin and a noble metal selected from the group consisting of silver, gold, platinum, palladium and rhodium, which noble metal is present in amounts from about 50% to about 1% of the bath thereby forming a tin alloy having a melting point below 700 C., and which noble metal forms at the operating temperature an oxide which is less thermodynamically stable than an oxide of tin.

3. In the manufacture of flat glass in ribbon form, advancing the glass over a molten alloy containing tin and silver containing from 1% to 50% of silver and having a melting point below 700 C.

4. A method according to claim 3, characterised by employing at least 1% of silver.

5. A method according to claim 3, characterised by employing at least 10% of silver.

6. A method according to claim 3, characterised by employing at least 30% of silver.

7. A method according to claim 3, characterised in that silver and tin are employed in equal proportions.

References Cited UNITED STATES PATENTS 3,083,551 4/1963 Pil kington 65 3,233,995 2/1966 JavauX 6565 DONALL H. SYLVESTER, Primary Examiner.

G. R. MYERS, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,342,574 September 19, 1967 Ronald Colgan Jewell It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3 line 17 "2 ,9ll ,758" should read 2 ,9ll 759 Signed and sealed this 7th day of October 1969.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer 

1. IN THE MANUFACTURE OF FLAT GLASS IN RIBBON FORM, ADVANCING THE GLASS OVER A MOLTEN ALLOY OF TIN AND A NOBLE METAL SELECTED FROM THE GROUP CONSISTING OF SILVER, GOLD, PLATINUM, PALLADIUM AND RHODIUM, WHICH NOBLE METAL IS PRESENT IN AMOUNTS FROM ABOUT 50% TO ABOUT 1% OF THE BATH AND IN A PROPORTION TO FORM A TIN ALLOY HAVING A MELTING POINT BELOW 700*C., AND FORMS AT THE OPERATING TEMPERATURE AN OXIDE WHICH IS MUCH LESS THERMODYNAMICALLY STABLE THAN AN OXIDE OF TIN AND MAINTAINING A PLENUM OF PROTECTIVE ATMOSPHERE OVER THE MOLTEN ALLOY. 